1. Field of the Invention
The present invention relates to a band pass filter circuit and a multilayer band pass filter preferably for use in high-frequency apparatuses.
2. Description of the Related Art
In mobile apparatuses, such as a notebook personal computer, a smart phone, a tablet terminal, and a cellular phone, a band pass filter is used as an electronic component that allows only signals in a predetermined frequency band to pass therethrough.
Examples of known fundamental configurations of band pass filters include an LC resonant circuit band pass filter in which an LC parallel resonant circuit is connected to a signal line that connects an input terminal and an output terminal to each other, as disclosed in International Publication No. WO2008/143071.
FIG. 12 illustrates the equivalent circuit of a multilayer band pass filter in which the existing LC-resonant-circuit-type band pass filter disclosed in International Publication No. WO2008/143071 is formed of two stages of LC parallel resonant circuits.
FIG. 13 illustrates the attenuation characteristics of the existing multilayer band pass filter illustrated in FIG. 12, obtained by simulation.
As can be seen from FIG. 13, in the attenuation characteristics of this multilayer band pass filter, the attenuation curve gradually falls on the high-frequency side outside of the passband. Hence, when there is another nearby high-frequency device whose passband is close to the above passband, mutual interference between the two will be generated in a frequency band with low attenuation, causing degradation in reception sensitivity.
To suppress the interference of high-frequency devices, a band pass filter with high attenuation outside of a passband is required. However, the band pass filter formed of the above-described circuit configuration is not able to meet market requirements in some cases.